All-optical valley switch and clock of electronic dephasing

TL;DR

This paper proposes an ultrafast all-optical method to switch valley polarization in 2D materials using attosecond pulses, enabling control beyond electronic decoherence times, with potential for valleytronic devices.

Contribution

It introduces a coherent control protocol for ultrafast valley switching and demonstrates how to measure electronic dephasing times using linearly-polarized pulses.

Findings

Demonstrated ultrafast valley switching in hBN and MoS₂ monolayers.

Proposed method to extract electronic dephasing time T₂ from valley Hall conductivity.

Achieved control faster than electronic and valley decoherence times.

Abstract

2D materials with broken inversion symmetry posses an extra degree of freedom, the valley pseudospin, that labels in which of the two energy-degenerate crystal momenta, K or K', the conducting carriers are located. It has been shown that shining circularly-polarized light allows to achieve close to $100\%$ of valley polarization, opening the way to valley-based transistors. Yet, switching of the valley polarization is still a key challenge for the practical implementation of such devices due to the short coherence lifetimes. Recent progress in ultrashort laser technology now allows to produce trains of attosecond pulses with controlled phase and polarization between the pulses. Taking advantage of such technology, we introduce a coherent control protocol to turn on, off and switch the valley polarization at faster timescales than electronic and valley decoherence, that is, an ultrafast optical valley switch. We theoretically demonstrate the protocol for hBN and MoS$_2$ monolayers calculated from first principles. Additionally, using two time-delayed linearly-polarized pulses with perpendicular polarization, we show that we can extract the electronic dephasing time $T_2$ from the valley Hall conductivity.